1. Field of the Invention
The present invention relates to an air bag device adapted to make a folded air bag expanded and deployed into a vehicle compartment by a gas generated by an inflator upon collision of a vehicle.
2. Description of the Related Art
An air bag adapted to be deployed into a vehicle compartment from a central portion of a steering wheel upon collision of a vehicle in order to restrain an occupant is generally formed by sewing two circular basic fabrics along their outer peripheries. The air bag is adapted to be expanded into a flat spherical shape by a gas generated by an inflator fixed in an opening provided in the center of one of the basic fabrics.
An upper body half of an occupant to be restrained by the air bag extends vertically, and hence the occupant-restraining performance can be enhanced by deploying the air bag first vertically and then laterally. In such a way, the order of folding the air bag has been conventionally devised in order to control the shape and direction of deployment of the air bag, as described above.
However, it is difficult to sufficiently control the shape and direction of the air bag in the process of deployment only by devising the order of folding the air bag, and the development of a further reliable technique has been desired.
A sewn portion of each basic fabric of the air bag is harder than the other portions of the air bag, due to its increased thickness where the basic fabric is superposed and due to an influence of the rigidity of a sewing thread. Therefore, in order to softly restrain an occupant in the process of deployment of the air bag, it is desirable to avoid the contact between the sewn portion and the occupant. For this purpose, it is necessary to deploy first the central portion of the air bag and then the outer periphery of the air bag.